What is a Battery Management System (BMS)?
The Battery Management System (BMS) for Ailisheng’s Mini and Compact Electric Construction Machinery is a technology specifically designed to oversee a battery pack. A battery pack is a component of battery cells that are electrically organized in a row x-column matrix configuration to deliver a target range of voltages and currents over time in response to an expected load profile.
The supervision provided by the BMS typically includes:
l It monitors the battery,
l provides battery protection,
l estimates battery operating conditions,
l continuously optimizes battery performance and reports operational status to external devices.
In this context, the term 'battery' implies the entire battery pack; however, monitoring and control functions are specifically applied to individual cells or packs called modules within the overall battery pack assembly. Lithium-ion rechargeable batteries have the highest energy density and are the standard choice for battery packs in many consumer products, from laptops to electric vehicles. While they are excellent performers, they can be quite unforgiving if operated outside of the normally tight Safe Operating Area (SOA), with results ranging from compromised battery performance to outright dangerous consequences. The role of a BMS is undoubtedly challenging, and the overall complexity and scope of oversight can involve a wide range of disciplines, including electrical, digital, controls, thermal, and hydraulics.
1. The functionality of battery management systems (BMS) is crucial for the effective operation of battery-powered equipment.
There is no universally applicable set of standards for BMS, with the technical design scope and realized features typically influenced by factors such as:
l cost, complexity, and size of battery packs;
l application of the battery and any safety, longevity, and warranty issues;
l certification requirements of various government regulations, costs, and penalties.
l If functional safety measures are not in place, these are critical.
There are many design features of a BMS, with battery pack protection management and capacity management being the two basic functions. We will discuss how these two functions work here. The two key areas of battery pack protection management are as follows: electrical protection, which means not allowing the battery to be damaged by use outside of the SOA; and thermal protection, which involves passive and/or active temperature control to maintain or bring the battery pack into the SOA.
1.1 Electrical Management Protection: Current
The most effective way to achieve electrical protection is to monitor the battery pack current and cell or module voltage. The electrical SOA of any battery cell is constrained by both current and voltage. Figure 1 illustrates a typical Li-Ion battery SOA, where a well-designed BMS will protect the battery pack by preventing operation outside the manufacturer's cell ratings. In many cases, further derating can be applied within the SOA safety zone to extend battery life.
Lithium-ion batteries have different charge and discharge current limits, and both modes can handle higher peak currents, albeit for brief periods. Battery manufacturers typically specify maximum continuous charge and discharge current limits, as well as peak charge and discharge voltage limits. A BMS that provides current protection will certainly apply the maximum continuous current. However, sudden changes in load conditions, such as the sudden acceleration of an electric vehicle, may be taken into account before this limit is reached. The BMS can incorporate peak current monitoring by integrating the current and, after a Δ time, deciding to either reduce the available current or interrupt the group current altogether. This allows the BMS to be almost instantaneously sensitive to extreme current spikes, such as short circuit conditions that do not attract the attention of any resident fuse, but can also tolerate high peak demands as long as they are not overdosed for too long.
1.2 Electrical Management Protection: Voltage
Please refer to Figure 2, which illustrates the range of voltages over which a lithium-ion battery must operate. It should be noted that these SOA boundaries will ultimately be determined by the inherent chemistry of the selected Li-ion battery and the temperature of the battery at any given time. Given the battery pack's exposure to significant current cycling, resulting from load demands and charging from diverse energy sources, these SOA voltage limits are likely to be further constrained to maximize battery longevity. The BMS must be cognisant of these limits and utilize this knowledge to make informed decisions based on the proximity of these thresholds. For instance, the BMS may request a gradual reduction of the charging current when approaching a high voltage limit or may request a complete termination of the charging current if that limit is reached. However, this limit is typically accompanied by additional inherent voltage hysteresis considerations to prevent control chatter about the shutdown threshold. Conversely, when approaching the low voltage limit, the BMS will request the critical active offending load to reduce its current demand. In the case of an electric vehicle, this can be achieved by reducing the allowable torque available to the traction motor. Naturally, the BMS must place the highest priority on driver safety considerations, while protecting the battery pack to prevent permanent damage.
1.3 Thermal Management Protection: Temperature
Lithium-ion batteries have a wide operating temperature range, but their overall capacity is reduced at low temperatures due to a significant decrease in the chemical reaction rate. In terms of capacity at low temperatures, they outperform lead acid and NiMH batteries; however, effective temperature management is essential as charging below 0°C (32°F) is physically challenging. During sub-zero charging, lithium metal can plate on the anode, resulting in permanent damage that reduces capacity and increases susceptibility to failure when subjected to vibration or other stressors. The BMS can regulate the battery pack's temperature through heating and cooling mechanisms.
The effectiveness of the thermal management system depends on various factors, including the size and cost of the battery pack, the performance objectives, the design criteria of the BMS, and the product cell, which may also consider the targeted geographical area. Regardless of the heater type, it is often more efficient to draw energy from an external AC power source or an alternative resident battery used to operate the heater when needed. However, if the electric heater has a moderate current draw, energy from the main battery pack can be siphoned off to heat itself.If a thermo-hydraulic system is used, an electric heater is used to heat the coolant that is pumped and distributed throughout the assembly.
There is no doubt that BMS design engineers have some tricks up their sleeve in the design industry to drip heat into the battery pack. One such strategy involves the activation of various power electronics within the BMS dedicated to capacity management. While this approach may not achieve optimal efficiency, its implementation is nevertheless beneficial. Cooling plays a crucial role in mitigating performance loss in lithium-ion battery packs. For instance, a battery might perform optimally at 20°C; however, if the package temperature is elevated to 30°C, its efficiency could decline by up to 20%. Additionally, if the battery pack is subjected to continuous charging and discharging at 45°C (113°F), the efficiency reduction could reach 50%. Furthermore, if the battery is exposed to excessive heat on a continuous basis, its lifespan may be reduced. This is especially the case during rapid charging and discharging cycles. Cooling is typically achieved by two methods: passive or active. Both technologies can be used. Passive cooling relies on the movement of airflow to cool the battery. In the case of an electric car, this means it is just traveling down the road. However, this can be more complex as airspeed sensors can be integrated to automatically adjust the deflector air dam to maximize airflow.At low speeds or when the vehicle is stopped, an active temperature-controlled fan may help equalize the battery pack with the ambient temperature, but this only raises the initial temperature of the pack in hot weather. A thermo-hydraulic active cooling system can be designed as a supplemental system, typically using a glycol coolant with a specified mixing ratio, which is circulated via electric pumps through pipes/hoses, distribution manifolds, cross-flow heat exchangers (radiators), and cooling panels against the battery pack components. The BMS monitors temperatures throughout the battery pack and opens and closes a variety of valves to maintain temperatures throughout the battery within a narrow temperature range to ensure optimal battery performance.
1.4 Capacity Management
Maximizing the capacity of a battery pack is arguably one of the most important battery performance features offered by a BMS. Without this maintenance, the battery pack may eventually become useless. The root of the problem is that battery pack 'stacks' (arrays of cells in series) are not exactly equal and inherently have slightly different leakage or self-discharge rates. While leakage is not considered a defect, it is a chemical characteristic inherent to the cells. However, it can be influenced by minor manufacturing variations. Initially, a battery pack may contain cells with close similarities, but over time, these cells may diverge due to various factors such as self-discharge, charge/discharge cycling, elevated temperatures, and general calendar aging. As previously discussed, while Li-Ion batteries generally perform well, they can be quite unforgiving if operated outside of a strict SOA. We have previously learned about the electrical protection required because lithium-ion batteries do not respond well to overcharging. Once fully charged, they cannot accept additional current; any excess energy is converted into heat, and the voltage can rise rapidly, potentially reaching hazardous levels. This is not a healthy situation for the cells and, if sustained, could cause permanent damage and unsafe operating conditions.
The battery pack series cell array determines the overall battery pack voltage and mismatches between neighboring cells can cause issues when attempting to charge any battery pack. Figure 3 shows why this is the case. If one has a perfectly balanced set of cells, then all is well because each cell will charge in an equal manner and the charging current can be cut off when the upper 4.0 voltage threshold is reached. However, in an unbalanced scenario, the top cell will reach its charging limit prematurely, necessitating the termination of the charging current in the branch before the bottom cells can be fully charged.
To illustrate this process, it is essential to define the state of charge (SOC) of a battery or module at a given time as the proportion of power available at full charge relative to the total power. Therefore, a battery at 50% SOC is 50% charged, which is similar to the quality factor of a power meter.BMS capacity management is all about balancing the change in SOC for each stack in the battery pack.SOC is not a directly measurable quantity, so it can be estimated by a variety of techniques, and the balancing schemes themselves typically fall into two broad categories: passive and active. There are many variations on the theme, and each type has advantages and disadvantages, so the BMS design engineer must decide which one is best suited to a given battery pack and its application. Passive balancing is the easiest to implement and explains the general balancing concept: the passive approach allows each cell in the battery pack to have the same charge capacity as the weakest cell. It uses a relatively low current to transfer a small amount of energy from the high SOC cells during the charging cycle so that all cells are charged to their maximum SOC. As illustrated in Figure 4, the BMS employs a sophisticated monitoring system that utilizes transistorized switches and appropriately sized discharge resistors in parallel with each cell. When the BMS detects that a cell is approaching its charging limit, it strategically directs excess current to the next cell in a top-down manner.
As illustrated in Figure 5, the before and after balancing process endpoints are as follows. In summary, the BMS balances the battery pack by allowing cells or modules in the pack to see a different charge current than the pack current in one of the following ways:
1). removing the charge from the most charged cells, which provides headroom for additional charge current to prevent overcharging and allows less charged cells to receive more charge current; or
2). Redirecting some or almost all of the charge current around the most charged cell, thus allowing the less charged cell to receive charge current for a longer period of time.
2. Types of Battery Management Systems
Battery management systems range from simple to complex, and can employ a variety of different technologies to fulfill their primary directive of 'taking care of the battery'.However, these systems can be categorized according to their topology, which relates to how they are installed and operated on cells or modules throughout the battery pack.
2.1 Centralised BMS architecture
The battery pack assembly features a central BMS, with all battery packs directly connected to it. Figure 6 illustrates the architecture of the centralized BMS, which offers several advantages, including increased compactness and cost efficiency due to the use of a single BMS. However, it is important to note that the centralized BMS also has certain disadvantages. Since all batteries are directly connected to the BMS, the BMS requires numerous ports to connect all battery packs, resulting in a substantial number of wires, cables, connectors, etc. in large battery packs, which complicates troubleshooting and maintenance.
2.2 Modular BMS Topology
A similar approach to centralized implementation sees the BMS divided into several repeating modules. Each module has a dedicated bundle of wires and is connected to adjacent sections of the battery pack (see Fig. 7). In some cases, these BMS sub-modules may be located under the supervision of the main BMS module. The function of the main BMS module is to monitor the status of the sub-modules and communicate with peripheral devices. The repetitive modularity facilitates troubleshooting and maintenance while accommodating expansion to larger battery packs with ease. The disadvantages of this approach are a slightly higher overall cost and, depending on the application, potentially duplicated unused functionality.
2.3 Primary/Secondary BMS
The concept is similar to the modular topology. In this case, the slave devices are limited to relaying measurement information only, whereas the master device is dedicated to computation, control, and external communications. As a result, the cost may be lower, as the slave devices tend to be simpler in function, have less overhead, and have fewer unused features.
2.4 Distributed BMS Architecture
The distributed BMS architecture is distinct from other topologies in which electronic hardware and software are encapsulated in modules connected to the battery via a connected harness. Distributed BMS integrates all electronic hardware on a control board placed directly on the cell or module being monitored. This approach eliminates the need for extensive wiring of multiple sensors and communication wires between neighboring BMS modules, enhancing the independence and autonomy of each BMS, which can perform calculations and communicate as required. However, despite this apparent simplicity, this form of integration does make troubleshooting and maintenance potentially problematic, as it is located deep within the shielded module assembly. Costs also tend to be higher because there are more BMSs throughout the battery pack structure.
3. The importance of a Battery Management System
Battery Management System(BMS) in ensuring functional safety during charging and discharging operations cannot be overstated. The BMS plays a crucial role in preventing the voltage, current and temperature of cells or modules under supervisory control from exceeding the specified SOA limits. Exceeding these limits for any duration can not only result in expensive battery packs being affected but also lead to dangerous thermal runaway conditions. Additionally, lower voltage threshold limits are meticulously monitored to safeguard the lithium-ion battery and maintain functional safety. It is imperative that lithium-ion batteries are maintained at the correct voltage levels. Failure to do so may result in the growth of copper dendrites on the anode, which can lead to increased self-discharge rates and potential safety concerns. The high energy density of lithium-ion powertrains is achieved at the expense of limited room for battery management errors. However, advancements in BMS and lithium-ion cells have contributed to the success and safety of this battery chemistry.
Battery pack performance represents the second most crucial function of the BMS, encompassing both electrical and thermal management. To optimize the overall cell capacity electrically, it is essential to balance all cells within the battery pack. This involves ensuring that the SOCs of neighboring cells throughout the assembly are approximately equal. This is pivotal as it not only enables optimal cell capacity but also prevents general degradation and reduces potential hotspots for overcharging weak cells. It is imperative to avoid discharging Li-ion batteries below the low voltage limit, as this may lead to memory effects and significant capacity loss. Electrochemical processes are highly sensitive to temperature, and batteries are no exception. As ambient temperature drops, battery capacity, and available energy decrease significantly. To address this, the BMS may engage an external in-line heater, such as one located on the liquid cooling system of an electric construction machine, or switch on a resident heating plate installed underneath a module of a battery pack within an electric vehicle or other electric device. Additionally, since charging lithium-ion batteries at low temperatures is detrimental to their lifetime performance, it is important to adequately raise the battery temperature in the first place. Most lithium-ion batteries are unable to be charged quickly at temperatures below 5°C and should not be charged at all at temperatures below 0°C.To ensure optimum performance during typical operational use, BMS thermal management often ensures that the battery is operated within the narrow Goldilocks operating region (e.g. 30-35°C). This protects performance, extends service life, and ensures a healthy, reliable battery pack.
4. The benefits of Battery Management Systems
Battery energy storage systems, often referred to as BESS can consist of dozens, hundreds, or even thousands of lithium-ion batteries strategically assembled together, depending on the application. These systems may be rated at less than 100V but may be as high as 800V, with battery pack supply currents ranging up to 300A or more. The consequences of mismanagement of a high-voltage battery pack can be catastrophic, even life-threatening, emphasizing the critical importance of a BMS in ensuring safe operation. The key benefits of a BMS can be outlined as follows:
4.1. Functional safety: This is especially crucial and necessary for large-sized lithium-ion battery packs, though even the smaller formats used in laptops can pose a significant risk of fire and damage. The personal safety of users of products containing lithium-ion power systems leaves little room for battery management errors.
4.2. Longevity and reliability. Battery pack protection management, electrical and thermal, ensures that all batteries are used within stated SOA requirements. This subtle oversight ensures the safe use of batteries and rapid charging and discharging cycles and inevitably produces a stable system that has the potential to provide years of reliable service.
4.3 Performance and range: The BMS battery pack capacity management, where inter-cell balancing is employed to equalize the SOC of neighboring cells on the battery pack assembly, allows optimum battery capacity to be achieved. Without this BMS functionality to account for variations in self-discharge, charge/discharge cycling, temperature effects, and general aging, the battery pack may eventually become useless.
4.4 Diagnostics, data collection and external communication. Supervisory tasks include continuous monitoring of all battery cells, where data logging can itself be used for diagnostics, but is typically used in computational tasks to predict the SOC of all cells in the assembly. This information is used in balancing algorithms but can be co-forwarded to external devices and displays to indicate available resident energy, estimate expected range or range/lifespan based on current usage, and provide a state of health for the battery bank.
4.5. Cost reduction and warranty. Introducing a BMS into a BESS increases costs, and battery packs are expensive and potentially dangerous. The more complex the system, the higher the safety requirements and therefore more BMS oversight is needed. However, the protection and preventive maintenance of the BMS in terms of functional safety, longevity and reliability, performance and range, and diagnostics ensures that it will reduce overall costs, including those associated with the warranty.
5. Conclusion
Simulation is an invaluable ally for BMS design, especially when applied to explore and solve design challenges in hardware development, prototyping and testing. With an accurate model of the lithium-ion battery, a simulation model of the BMS architecture is recognized as an executable specification for virtual prototyping. In addition, simulations allow painless investigation of variants of BMS supervisory functions for different battery and environmental operating scenarios. Implementation issues can be identified and investigated very early, which allows improvements in performance and functional safety to be verified prior to implementation on actual hardware prototypes. This reduces development time and helps to ensure that the first hardware prototype is robust. In addition, when performed in an embedded system application, many identity verification tests can be performed on the BMS and battery pack, including worst-case scenarios.
